1. Field of the Invention
This invention relates to a method for separating solids from a hydrocarbon slurry. This invention particularly relates to a method for separating solids from a hydrocarbon slurry using an additive which includes a polymer.
2. Background of the Invention
Separating solids, particularly finely divided solids, from a slurry containing a fluid or liquid and such solids is needed for many different material productions either directly from natural sources or in manufacturing plants. For example, in a fluid catalytic cracking (FCC) unit, zeolitic catalysts in a fluidizable form, i.e. finely-divided particles with certain defined particle size distributions, are used to effect cracking of heavy petroleum fractions into lighter hydrocarbon products at elevated temperatures. Due to the severe reaction conditions, even the most refractory silicoaluminum oxide type molecular sieve catalysts could suffer some attrition to produce additional fine particles. Regardless the source of the finely divided particles, some of them are easily carried into the product stream. These particles need to be removed before the products can be processed further. This product stream from an FCC unit is referred to hereinafter as "slurry oil."
Another example where solids need to be separated from products is catalytic conversion of synthesis gas (syn gas), a mixture comprising primarily hydrogen and carbon monoxide, to hydrocarbons and oxygenated products. This type of reaction is commonly referred to as a Fischer-Tropsch (F-T) synthesis reaction. It is frequently carried out in a liquid slurry system with finely divided solid catalysts or in a liquid system with a homogeneous catalyst. Even with a homogeneous catalyst, it is not unusual to observe catalyst particles or other solids precipitating out of the reaction system due to decompositions or other chemical changes of the catalyst during reaction. The catalyst particles need to be separated from the solvents and reaction products as part of the purification process. If preferred, the recovered catalyst particles can be recycled for reuse, reclamation of precious metals or disposal of as waste. The solid-free product stream is then processed further.
Solids separation is also important for naturally occurring formation fluids such as crude oil, bottoms from various oil refining processes, residue and numerous streams from chemical or polymer plants. All of these streams are known to contain different types and varying amounts of finely divided solid particles. These finely divided solid particles could be inorganic materials such as sand or dirt or catalyst, organic compounds, or mixtures of organic, inorganic and organometallic compounds. The particles could exist in a wide range of sizes. These solid particles need to be separated from other products as part of the purification step. Recovery and production of minerals or metals may also require such separations of solids from an aqueous phase.
Many different methods and equipment have been used to separate, remove or recover the finely divided solids from a variety of slurry mixtures as discussed in the foregoing examples. These methods and equipment include sedimentation, magnetic separation if the particles are magnetic, and/or use of processing equipment such as hydrocyclones and centrifugal separators. In processes where direct physical/mechanical separations are not economical, technically feasible or fast enough, different chemicals have been used to effect, aid and/or accelerate settling of finely divided solid particles upon standing, storage, centrifugation or other ways. For instance, U.S. Pat. No. 5,481,059 discloses the use of an adduct between alkylphenolformaldehyde resin alkoxylate compound and polyacrylic acid to aid settling of solids. U.S. Pat. No. 5,476,988 discloses a method of accelerating settling of finely divided solids in hydrocarbon fluids by adding a certain quaternary fatty ammonium compound to the slurry.
To be effective, it is generally desirable to have chemical aids, additives and/or polymers that are large, easy to separate and/or capable of forming strong interactions with the finely divided solids present in the slurry. Such strong interactions may be chemical, physical, electrostatic, van der waals, or a combination thereof. It is also desirable to form a sludge or other forms of precipitation between the solids and the additive that are readily separable from the fluid or liquid phase of the slurry. It would be advantageous to accelerate the settling of the finely divided solids to shorten the settling time required to achieve the desired level of residual solids in the fluid/liquid phase. This would help reduce the size of the settling tank or other related equipment and/or increase the throughput of the process. It would be a further advantage if these chemical aids, additives or polymers are inexpensive or more effective than those already known.
It was unexpectedly discovered that a number of large polymers can effect settling or accelerated settling of finely divided particles when they are used as part of an additive in accordance with the present invention. The present invention is particularly useful for separating and settling finely divided solids, such as FCC catalyst, from FCC slurry oils.